1. Technical Field
This invention relates to the field of variable gain amplifiers. In particular, the invention relates to variable gain amplifiers for low voltage and high frequency operation.
2. Description of the Prior Art
A variable gain amplifier (VGA) is often used in automatic gain control (AGC) applications and often needs to cover a gain range where it is attenuating, with a gain of less than 1, and amplifying, with a gain of greater than 1. Modern integrated circuit applications for VGA circuits are requiring increased operating frequencies, low supply voltages and relatively high input signal levels (at the low gain settings).
U.S. Pat. No. 6,512,416 to Burns et al. discloses a variable gain amplifier for use in a cable television system. An extended range of variable gain amplifier is achieved switching in parallel amplifiers in an architecture. There is no gain adjustment within the individual amplifier blocks of the architecture, the gain adjustment is achieved by switching amplifiers in and out of circuit. There is mention of varying the gain by adjusting the tail current which has several disadvantages, notably the maximum output swing is also changed and the frequency performance varies a lot.
The '416 patent also requires a large number of amplifier blocks which results in a high capacitive loading at the summing node which requires the addition of inductors. The need to use such components on a chip is undesirable due to the large silicon area that they occupy, increasing the cost of the solution.
Serial link technology is becoming pervasive in the majority of system to system, system to peripheral or local inter computer connections, replacing parallel bus connections. A variable gain amplifier is an important part of serial link technology and a solution is particularly required for high frequency applications where high input voltages can be processed with the low supply of sub-micron CMOS (complementary metal oxide semiconductor) integrated circuit processes. Therefore, it is an aim of the present invention to provide a variable gain amplifier solution suited for low voltage and high frequency applications.